Overview
# Pick a Mythological or Historical Figure
# Pick a Class
# Statistics!
# Skills!
# Noble Phantasms!
# Noble Phantasms, Again
# Balancing
# Appearance

STEP ONE:
* Every Servant has done something. No-one gets in by mere dint of existing as a ruler, or being quietly assassinated whilst ruling a vast swathe of land. Such a person would have no Noble Phantasm, no Personal Skills. Boring, in other words.
* Avoid the Book of Genesis. In fact, try and stay away from Creation Myths in general. The issue with them (where people are involved) can be summed up in one word: Gilgamesh. We know that he's the Oldest King (it's one of his titles) and we know that he has the originals of nigh on everything. Biblical myths are even worse (Gilgamesh, in his own lifetime, met Utnapishtim, and Utnapishtim and Noah cannot coexist, unravelling things backwards). It would require a lot of work, and result in a Servant that cannot physically possess anything that would have spawned from that treasury.
* Ensure they're sufficiently human to start with. Medusa is a Divine Spirit turned monster, and is pretty much as inhuman as you're going to get, regardless of how she looks. At no point should outright deities or things like Cerberus be summoned, they just don't make sense as Heroic Spirits.

STEP TWO:
* Berserker is named such for berserk rage. Not for a short temper, for going berserk in battle. Lancelot had a huge body count and killed his friend's brothers out of desperation, Heracles kept flipping and killing things, part of Lancer's myth is all about how he was a berserker.
* Match the class to their skills in life. Failing an exact match, fudge things a little. Just don't expect them to be the best Saber ever if they didn't fit to start with.

STEP THREE:
* Stats match deeds. Those who have no notably awesome feats of strength would not have abnormal strength for their class. Assassin should not be going around smashing the scenery with near misses unless you've gained Heracles.
* Compare with the various strongest Servants. Saber with Rin's stats, Gilgamesh as summoned by Tokiomi, and Heracles. Use common sense: if the character summoned is, say, Boudicca, there is no reason that she would be as physically able as them. One is 2/3 demigod and one of the oldest heroes, one is pretty much the most accomplished hero ever, and the third makes up for any physical deficits with Prana Burst and nigh infinite mana. More mundane medieval knights should be relying on guile against heroes from the Age of Gods, not overpowering them.

STEP FOUR:
* Class skills: easy. Magic Resistance B if they've dealt with it in life, A if they've got another reason for immunity. Presence Concealment A+ for those who're never found.
* Skill Number: Lancer has 4, minus Divinity (which is best discounted for these things, as it does... nothing). One is an A-rank. 5 A-ranked skills is too far.
* Skill Rank: Iskander, for all that his onward conquest was forced to a halt by his soldiers' lack of desire to continue and his later ability to summon them as a Servant, has Military Tactics B. Military Tactics A would need something amazing. Keep these in mind.
* Eye of the Mind (False) is natural intuition to overcome handicaps, (True) is planning and analysis. One is innate, the other is learned from initial skill amplified by experience. Instinct seems to be a lesser form of Eye of the Mind (False). Make sure to pick the right one: Achilles, for all his martial brilliance, would be more likely to have (False) or Instinct, he hardly lived through enough battles to pick up (True).
* MAKE THINGS UP. So, you want a guerilla tactics ability? Write one, don't combine disengage and presence concealment. Not only does it prevent broad, easily abused combinations, it gives a Servant some distinctiveness. Just remember to limit it.
* Don't copy other Servants' signature abilities without really good reason. Especially things such as Golden Rule, where the A-rank is closely intertwined with Gate of Babylon.

STEP FIVE:
* Basic term overview: anti-unit will only hurt one person, anti-army will always damage anyone caught nearby, anti-fortress is certain to devastate your surroundings. These are statements of area of effect. Power of a Noble Phantasm is given by rank, not type.
* Think very, very carefully about giving anything an EX-Rank. Canonical EX-rank stuff is essentially 'I win'. If you do give a character something like this, try and make it because it's unrankable due to not qualifying as a normal Noble Phantasm than overwhelming power.
* Almost every Noble Phantasm has some physical component, however minor. God Hand and Cybele are built into their users' bodies, Blood Fort Andromeda has its basis as magecraft. The exceptions? Reality Marbles, which are vanishingly rare (one requires the willpower of thousands of Servants, the other stems from having Avalon shoved in his chest for ten years, and the possible third is also 'summon theatre') and more strongly tied to Dead Apostle Ancestors than anything else (well, Crimson Moon, but it's a minor quibble)

STEP SIX:
* Always go for the simplest option first: check out techniques or items. Upgrade, rank, and designate an area as appropriate.
* Second option is to pair with deeds and give something that embodies them. Flying Lightning Chariot from cutting the knot? Well, the chariot existed, and the bulls attached were dedicated to Zeus. To take a quick example: Julius Caesar, crossing the Rubicon. A Noble Phantasm represented by his gladius, it embodies refusing to dissolve his armies and therefore passing the point of no return. Remove any negative penalties to his statistics and temporarily grant Battle Continuation A. If the fight results in a draw, Caesar still dies.
* Count them carefully. Excepting Gilgamesh, whose Noble Phantasm is to own Noble Phantasms (and Zealot, who was a joke), the most possessed... is five, and some of those are highly situational (Perseus). Next up is Medusa, who effectively has four, one of which is her very body, plus a Noble Phantasm-level ability to summon Pegasus. Three is the normal maximum.
* Link to their past. If in doubt, fudge history.
* NO FULL-STRENGTH SERVANT SUMMONING, EVER. No summoning near-Servant level armies, either. The power to massively multiply your power falls under the 'careful with EX-ranks' thing.

STEP SEVEN:
* Read through at the end and check how your abilities combine. Individual elements can add up to unavoidable combos. An ability that is immensely powerful only if used at the beginning of a fight, ranks in Disengage (which resets things to initial conditions), and A or A+ ranked Agility form a broken combination.
* A similar sort of situation goes for being immune to classes of Noble Phantasms (such as anti-army) but having the perfect skill-set for dealing with anti-unit attacks.
* External conditions that aren't limitations. Only works at nighttime? Sums up the conditions for normal Grail Wars. Requires a city? How many Grail Wars don't happen in one? If the only balancing on an item are the limitations, make sure the limitations aren't casually fulfilled.
* No True Magic. Ever. Medea is honestly dangerous enough without getting to that level. True Magic leads to the same problems as EX-Ranked Noble Phantasms: it has been known to kill the moon and... summon Servants. Even a flawed copy of the Second Magic can OHKO Servants without pause for recovery.

STEP EIGHT:
* Though tantalising, evidence has shown no Servant summoned in armour will have a chainmail bikini. They have actual armour.
* Huge breasts on an athletic warrior are just nonsensical. They're mostly fat. Somewhere here, logic fails.
* A+ ranked strength on feminine children is also a problem.
* In short: keep it appropriate.